An evolved packet system (EPS) provides both packet switched service (PSS) and non-packet switched service (NPSS). An example of an NPSS is a circuit switched fallback (CSFB) feature where, even though signaling for a circuit switched (CS) service request is performed over the EPS, a wireless transmit/receive unit (WTRU) is directed to a legacy network where the actual call set-up signaling and user data functions are provided. Another example of NPSS is the provisioning of a short message service (SMS) over an EPS control plane.
It is also possible to provide SMS over an Internet protocol (IP) network, such as the EPS network, but in this case the SMS is simply carried over the user/data plane of the EPS and is not considered an NPSS.
However, in order to provide NPSS, a mobility management entity (MME) in an long term evolution (LTE) network uses an ‘SGs’ interface with the mobile switching center (MSC)/visitor location register (MSC/VLR). Furthermore, the WTRU registers for both PSS and NPSS, (i.e., the WTRU is attached to both packet switched (PS) and CS domains), either by performing a combined EPS/international mobile subscriber identity (IMSI) attach procedure, or by performing a combined tracking area (TA)/location area (LA) update procedure. For the WTRU to perform a combined registration, it has to know, a priori, that the MME to which it is registered, or is about to register to, has an SGs interface with the MSC/VLR.
Currently, it has not been standardized whether the network broadcasts or advertises the existence of the SGs interface, (e.g., using system information (SI) messages), or whether a WTRU that accesses both PS and NPSS always requests a combined registration. Thus, it is not clear how a WTRU that supports CSFB and/or generic CS over EPS services, (possibly with a different network architecture), can determine whether messages that it receives, (either paging in idle mode or non-access stratum (NAS) messages in connected mode), are associated with CSFB or CS over EPS.
Based on the above, a method and apparatus are desired that provide non-packet switched service in a target RAT network to differentiate between messages that are associated with CSFB and messages that are associated with CS over EPS, without overloading the signaling or the system requirements.